526 PRACTICAL ZOOLOGY. 



well as the generative apparatus present differences 

 in the two sexes, being modified for the performance 

 of special functions in the sexual life, such as the 

 differences in color and markings of male and female 

 butterflies, and the differences between the wings of 

 male and female crickets, already noticed. These 

 differences are known as secondary sexual characters. 

 Compare the male and female crayfish as regards the 

 width of the abdomen. 



Study carefully the structure and action of the tail- 

 fin. Its middle piece is the tel son, underneath which 

 is the external opening of the intestine, the anus. 



Remove the telson, and without disturbing the side 

 parts of the tail-fin, separate the sixth abdominal ring 

 from the fifth. Now carefully compare this (sixth) 

 ring and its appendages with the third ring and its 

 appendages. 



Are the appendages of the thorax borne upon rings 

 like those of the abdomen ? If so, where are the 

 rings? With forceps seize the base of one of the 

 hindmost pair of walking legs, and move it back- 

 wards and forwards; are these borne on a distinct 

 ring? Carefully clean the sternum between the other 

 walking legs, and look closely for indications of rings. 



5. With the forceps break away one side of the carapace, 

 beginning at the lower edge. This lays bare the 

 white, feathery gills. Move the legs of this side 

 back and forth, watching the gills. 



6. Study now the hindmost of the walking, or thoracic 

 legs. Count its segments. Observe how the first 

 segment is joined to the body. Flex the leg as far as 

 possible, in every direction, noting the number of 



